The present invention refers to a mixer for double dispensing cartridges or dispensing appliances having at least two storage cylinders whose volumetric ratio is different from 1:1, comprising a mixer housing which is attachable to the cartridge respectively the dispensing appliance, and a mixer element group arranged therein, the two contiguous dispensing openings of the cartridge resp. the dispensing appliance communicating with the inlet portion of the mixer element group. Such mixers are known e.g. from U.S. Pat. No. 4,538,920, and they have the function of intimately mixing the two components dispensed from the dispensing cartridges n order to prepare them for their application.
If the two storage cylinders of the dispensing cartridge have approximately identical volumes and also have approximately equal dispensing openings, no major problems are involved in mixing the two components from the start. If however the two storage cylinders have substantially different volumes--which is equivalent to different cross-sectional areas since the storage cylinders have the same length on account of manufacturing and operating conditions--and different dispensing openings, e.g. of a ratio of 10:1, there is a risk that the component from the larger storage cylinder fills up the mixer element group of the mixer at the beginning of the dispensing process and that the second component having a smaller volume is thus suppressed or subdued. In particular, in the inlet area, the larger partial flow "A" will press the smaller partial flow "B" against the wall of the mixing tube on the B side, where it is retarded by the wall friction and therefore subdued or suppressed by the larger A component and thus can only begin to flow after some time. In this initial phase, the partial flow A precedes, and a mixture according to the intended proportions cannot take place, so that the components cannot or only partially react with each other and their use results in defective bonds, joints, impressions, etc.
Since the second component is generally the hardener (catalyst) and a component mixture with an insufficient amount of hardener is useless, it is common practice, in order to stabilize the desired proportions of the partial flows, to dispense a certain quantity which is discarded before being able to start the actual application of the mixture. Besides the loss of component substance, these dispensed substances cannot harden and may cause disposal problems.
In European Patent Publication No. 0 584 428, a solution of this problem is suggested by first retaining the first, larger component in a chamber and by providing means for directing this first component to the inlet chamber of the second component and for carrying the latter along. Although this embodiment provided an improvement with respect to the state of the art, this solution results in an increased flow resistance due to several necessary restrictions of the cross-sectional area. This increased flow resistance may require a reinforcement of the cartridge walls and involves higher dispensing forces.
Furthermore, a dispensing appliance is known from European Patent Publication No. 472,448 in which the outlets of the dispensing appliance resp. of the cartridge housing are arranged in such a manner that the larger component flows around the smaller one and carries it along. The appliance is intended for use with or without a mixer. In the case where no mixer is used, the two components unite in the cartridge or in the dispensing appliance already, and because of the hardened substance, the outlet area must be cleaned after a cartridge exchange before continuing to work. If a mixer is used, the substance hardens there, and after exchanging the mixer, the work can be continued without cleaning the outlet area of the cartridge or of the dispensing appliance. However, in the above appliance, no measures are taken to ensure an exact mixing ratio.